Every conventional system of the kind referred to above includes the following elements: a tank; a pump; a metering device for metering fuel to the intake air; a feed line for feeding the fuel from the pump to the metering device; and, a control unit for driving the pump and the metering device.
Motor vehicles equipped with internal combustion engines adapted to burn different fuels such as gasoline or gasoline/methanol mixtures are usually equipped with a sensor for measuring the composition of the pumped fuel. The signals of such a sensor make it possible to adapt precontrol values for the metering of fuel to the particular fuel composition. Precontrol values are applied on a test stand for a pregiven type of fuel such as gasoline. During operation of the engine, these precontrol values such as injection times are adapted with the aid of an adaptation to the particular operating conditions. The adaptation to the particular fuel composition takes place with the aid of the signal of the above-mentioned sensor which measures the fuel composition. If injection times for the use of gasoline are applied, then these injection times should be extended, for example by approximately 50%, when the above-mentioned sensor announces that the actual tank content comprises approximately 50% gasoline and 50% methanol.
It is a continuous effort to use as few sensors as possible in the area of motor vehicle electronics. Accordingly, various suggestions have been made to eliminate the above-mentioned sensor for detecting the fuel composition. U.S. Pat. No. 5,094,208 discloses a method wherein the control deviation in the lambda control loop is measured after each tank refill and an adaptation value is so changed that the detected control deviation is eliminated. This method has the disadvantage that it can only function when the lambda control is active; however, this is not the case especially for cold engines. However, even for warm engines, the method is problematical because of the abrupt change of the adaptation value since this procedure can easily lead to control oscillations.
U.S. Pat. No. 5,121,732 discloses various methods which operate to render even cold engines operationally ready when the fuel composition has changed greatly after a tank refill, for example, when a tank containing gasoline has been driven until almost empty and then filled with a fuel comprising for the most part methanol. The fuel composition present is estimated with the aid of the tank conditions before and after tanking and upon the basis of the data of the fuel to be purchased. The precontrol values are then changed for the operation of the engine with fuels of the possible compositions and an investigation is made with which composition the machine will best operate. Control is continued then with these values. Independently of whether the method is applied to a cold engine or not, an adaptation with a short time constant is carried out when the lambda control is released and after filling the tank.
These methods too however present problems for example because the above-mentioned rapid adaptation brings about a slight tendency of control oscillations but nonetheless not so intense as with the method according to the above-mentioned U.S. Pat. No. 5,094,208 wherein an abrupt change of the adaptation value takes place. Poor engine running can develop when operating the engine with values for the different possible compositions when control is made with just those estimated values which lie farthest from those which are applicable for the fuel composition which is actually present.